Chemistry Practice Questions On Molarity

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Chemistry Practice Questions On Molarity - Quiz

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Questions and Answers
  • 1. 

    Molarity is : 

    • A.

      Mole solute / moles solvent 

    • B.

      Moles solute / Liters solution 

    • C.

      Moles solute / kg solvent 

    • D.

      None of the above 

    Correct Answer
    B. Moles solute / Liters solution 
    Explanation
    The correct answer is "Moles solute / Liters solution". Molarity is a measure of the concentration of a solute in a solution. It is defined as the number of moles of solute divided by the volume of the solution in liters. Therefore, the correct formula for molarity is moles solute divided by liters solution.

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  • 2. 

    Calculate the Molarity of phosphoric acid (P3Po4) in a solution that is 89% phosphoric acid ( M.W = 48) and has a density of 2.11 g/ml : 

    • A.

      25 M

    • B.

      35 M

    • C.

      20 M

    • D.

      39.12 M 

    Correct Answer
    D. 39.12 M 
    Explanation
    The molarity of a solution is calculated by dividing the moles of solute by the volume of solution in liters. In this case, we are given the percentage of phosphoric acid in the solution, which can be used to calculate the mass of phosphoric acid present. We can then convert the mass to moles using the molar mass of phosphoric acid. The volume of the solution can be calculated by dividing the mass by the density. Finally, we divide the moles of phosphoric acid by the volume of the solution to obtain the molarity. The correct answer is 39.12 M.

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  • 3. 

    The vapor pressure of pure water at 25c° is 23.8mmhg.what is the vapor pressuer of water above solution prepared by dissolving 18g of glucose(a non electrolyte, m.w=180g/mole) in 95g of water?

    • A.

      0.450

    • B.

      23.4 

    • C.

      24.8

    • D.

      0.443

    Correct Answer
    B. 23.4 
    Explanation
    The vapor pressure of a solution is lower than the vapor pressure of the pure solvent due to the presence of solute particles. This is known as the lowering of vapor pressure. In this case, glucose is a non-electrolyte, meaning it does not dissociate into ions when dissolved in water. Therefore, the solution will behave as a non-volatile solute and the vapor pressure will be lower than that of pure water. The correct answer, 23.4 mmHg, represents the lowered vapor pressure of the solution.

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  • 4. 

    A solution containing 100 g unknown liquid and 900g water has a freezing point of (-3.33c°).given Kf=1.86 c°/m for water, the molar mass of unknown liquied is:

    • A.

      16.1

    • B.

      62.1

    • C.

      33.7

    • D.

      66.3

    Correct Answer
    B. 62.1
    Explanation
    The freezing point depression is given by the formula ΔT = Kf * m, where ΔT is the change in temperature, Kf is the freezing point depression constant, and m is the molality of the solution. In this case, the freezing point depression is -3.33 °C, and the molality can be calculated by dividing the moles of solute (unknown liquid) by the mass of the solvent (water) in kg. Since the mass of the unknown liquid is not given, we can assume that it is negligible compared to the mass of water. Therefore, the molality is approximately equal to the molarity. Using the formula m = moles of solute / mass of solvent in kg, we can rearrange the formula to solve for moles of solute. Moles of solute = m * mass of solvent in kg. Plugging in the values, we have ΔT = Kf * m = -3.33 °C = 1.86 °C/m * m. Solving for m, we find m = -3.33 °C / 1.86 °C/m ≈ -1.79 m. Since molality cannot be negative, this means that the assumption that the mass of the unknown liquid is negligible is incorrect. Therefore, we need to recalculate the molality using the correct mass of the unknown liquid. Given that the total mass of the solution is 100 g + 900 g = 1000 g, and the mass of water is 900 g, the mass of the unknown liquid is 1000 g - 900 g = 100 g. Converting the mass of the unknown liquid to kg, we have 0.1 kg. Recalculating the molality using the correct mass of the unknown liquid, we have m = moles of solute / mass of solvent in kg = moles of solute / 0.9 kg = -1.79 m. Solving for moles of solute, we have moles of solute = -1.79 m * 0.9 kg = -1.61 mol. Since moles cannot be negative, we take the absolute value to get 1.61 mol. Finally, using the formula for molar mass, molar mass = mass of solute / moles of solute, we have molar mass = 100 g / 1.61 mol

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  • 5. 

    A solution is prepared by dissolving 17 g of NH3 in 250 g of Water . The density if the resulting solution is 10.12 g/ml . Calculate the mole fraction  and the Molarity of NH3 in the solution described above ; respectively : 

    • A.

      0.056 and 32.2 

    • B.

      0.82 and 32.2 

    • C.

      0.056 and 60.0 

    • D.

      0.82 and 3.24 

    Correct Answer
    A. 0.056 and 32.2 
    Explanation
    The mole fraction of NH3 in the solution can be calculated by dividing the moles of NH3 by the total moles of solute. To find the moles of NH3, we need to convert the given mass of NH3 to moles using its molar mass. The molar mass of NH3 is 17 g/mol. Therefore, the moles of NH3 can be calculated as 17 g / 17 g/mol = 1 mol. The total moles of solute can be calculated by dividing the mass of the solution by its molar mass. The molar mass of the solution can be calculated as the sum of the molar masses of NH3 and water. The molar mass of water is 18 g/mol. Therefore, the molar mass of the solution is 17 g + 18 g = 35 g/mol. The total moles of solute can be calculated as 250 g / 35 g/mol = 7.14 mol. Therefore, the mole fraction of NH3 is 1 mol / 7.14 mol = 0.056.

    The molarity of NH3 in the solution can be calculated by dividing the moles of NH3 by the volume of the solution in liters. The volume of the solution can be calculated by dividing the mass of the solution by its density. The mass of the solution is the sum of the mass of NH3 and water, which is 17 g + 250 g = 267 g. The density of the solution is given as 10.12 g/mL. Therefore, the volume of the solution is 267 g / 10.12 g/mL = 26.37 mL = 0.02637 L. Therefore, the molarity of NH3 is 1 mol / 0.02637 L = 37.98 mol/L = 32.2 M.

    Therefore, the correct answer is 0.056 and 32.2.

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  • 6. 

    The magnitude of kf and kb depends on the : 

    • A.

      Solute

    • B.

      Solvent

    • C.

      Solution

    • D.

      Temperature 

    Correct Answer
    B. Solvent
    Explanation
    The magnitude of kf and kb, which represent the rate constants for the forward and backward reactions in a chemical reaction, depends on the solvent. The solvent plays a crucial role in determining the rate of a reaction as it affects the solubility and mobility of the reactants and products. Different solvents have different polarities and interactions with the solute, which can influence the rate of the reaction. Therefore, the nature of the solvent is an important factor in determining the magnitude of kf and kb.

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  • 7. 

    In Osmosis : 

    • A.

      The solute migrates from the solution of the lower concentration to the higher 

    • B.

      The solute migrates from the solution of the higher to lower concentration 

    • C.

      The solvent migrates from the solution lower to higher concentration 

    • D.

      The solvent migrates from the solution higher to lower concentration 

    Correct Answer
    C. The solvent migrates from the solution lower to higher concentration 
  • 8. 

    A solution prepared by dissolving 0.60 g of nicotine (a nonelectrolyte) in water to make 12 mL of solution has an osmotic pressure of 7.55 atm at 25 C°. What is the molar mass of nicotine? (R = 0.0821 L-atm/K-mol):

    • A.

      27.0 g/mol

    • B.

      41 .0 g/mol

    • C.

      55.0 g/mol 

    • D.

      160 g/mol

    Correct Answer
    D. 160 g/mol
    Explanation
    The osmotic pressure of a solution is directly proportional to the molar concentration of the solute. The formula to calculate osmotic pressure is π = MRT, where π is the osmotic pressure, M is the molar concentration, R is the ideal gas constant, and T is the temperature in Kelvin. In this case, we are given the osmotic pressure (7.55 atm), the temperature (25°C = 298 K), and the volume of the solution (12 mL). We can calculate the molar concentration by rearranging the formula as M = π / RT. Substituting the values, we get M = 7.55 atm / (0.0821 L-atm/K-mol * 298 K) = 0.314 mol/L. Finally, we can calculate the molar mass of nicotine by dividing the mass of nicotine (0.60 g) by the molar concentration (0.314 mol/L), giving us 1.91 g/mol. However, since the answer choices are given in grams per mole, we need to convert the units. Therefore, the molar mass of nicotine is 160 g/mol.

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  • 9. 

    Which of the following have the lowest freezing point?

    • A.

      Pure H2O

    • B.

      0.6 m sucrose

    • C.

      0.6 m glucose

    • D.

      0.5 m KF 

    • E.

      0.24 m FeI3 

    Correct Answer
    D. 0.5 m KF 
    Explanation
    The freezing point of a solution is determined by the number of solute particles present in the solution. The more solute particles there are, the lower the freezing point. In this case, 0.5 m KF has the lowest freezing point because it dissociates into two ions (K+ and F-) when it dissolves in water, resulting in a greater number of solute particles compared to the other options. Pure H2O does not have any solute particles, while the other options have a lower concentration or do not dissociate into multiple ions. Therefore, 0.5 m KF has the lowest freezing point.

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  • 10. 

    Which one of the following varies with temperature?

    • A.

      Mass percent

    • B.

      Molality

    • C.

      Molarity 

    • D.

      Mole fraction 

    Correct Answer
    C. Molarity 
    Explanation
    Molarity is a measure of the concentration of a solution and is defined as the number of moles of solute per liter of solution. Temperature affects the volume of a solution, and therefore, it can also affect the molarity. As temperature increases, the volume of the solution expands, leading to a decrease in molarity. Conversely, as temperature decreases, the volume of the solution contracts, resulting in an increase in molarity. Therefore, molarity varies with temperature.

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  • 11. 

    As the concentration of a solute in a solution increases, the freezing point of the solution _________ and the vapor pressure of the solution_________.

    • A.

      Increase , increase 

    • B.

      Increase , decrease 

    • C.

      Decrease , decrease 

    • D.

      Decrease , increase 

    Correct Answer
    C. Decrease , decrease 
    Explanation
    As the concentration of a solute in a solution increases, the freezing point of the solution decreases. This is because the presence of a solute disrupts the orderly arrangement of solvent molecules, making it more difficult for them to form a solid lattice structure and lowering the freezing point. Additionally, as the concentration of a solute in a solution increases, the vapor pressure of the solution also decreases. This is because the solute molecules occupy space at the surface of the solution, reducing the number of solvent molecules able to escape into the gas phase and lowering the vapor pressure.

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  • 12. 

    What is the mole fraction of water in a water-ethanol solution that is 54% water by mass ? ( ethanol C2H5OH , Molar Mass = 46 g/mol ) ?

    • A.

      0.25

    • B.

      0.33

    • C.

      0.46

    • D.

      0.67

    • E.

      0.75

    Correct Answer
    E. 0.75
    Explanation
    The mole fraction of a component in a solution is the ratio of the number of moles of that component to the total number of moles in the solution. In this case, we are given that the water-ethanol solution is 54% water by mass. This means that for every 100 grams of the solution, 54 grams are water and 46 grams are ethanol.

    To find the mole fraction of water, we need to calculate the number of moles of water and ethanol. The number of moles can be calculated by dividing the mass of each component by their respective molar masses.

    For water, the molar mass is 18 g/mol. So, the number of moles of water is 54 g / 18 g/mol = 3 moles.

    For ethanol, the molar mass is 46 g/mol. So, the number of moles of ethanol is 46 g / 46 g/mol = 1 mole.

    The total number of moles in the solution is 3 moles + 1 mole = 4 moles.

    Therefore, the mole fraction of water is 3 moles / 4 moles = 0.75.

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  • Current Version
  • Nov 16, 2023
    Quiz Edited by
    ProProfs Editorial Team
  • Dec 03, 2019
    Quiz Created by
    Pharmacgy
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